1. Field of the Invention
The subject invention is directed to a process for preparing alkyl nitrites, particularly methyl nitrite and ethyl nitrite. More particularly, the subject invention is directed to a process for removing impurities from an alkyl nitrite production zone gaseous product stream in an integated cycle wherein the production of alkyl nitrite and subsequent conversion of alkyl nitrite to dialkyl oxalate are coupled.
2. Description of Related Art
Alkyl nitrites, i.e., esters of nitrous acid, have been found useful in a variety of areas including additives to motor fuels, stabilizers for vinyl compounds such as spasmolytic agents, reagents for diazotization and reagents for chemical synthesis. Processes for preparing alkyl nitrites can be found, inter alia, in U.S. Pat. Nos. 4,229,591; 4,353,843 and 4,629,806 and in Japanese Application No. 53-8268. The process for forming alkyl nitrites (referred to herein as the nitrite process) may be understood more fully by reference to the following equations:
(1) 2NO+O.sub.2 .fwdarw.2NO.sub.2 PA0 (2) NO.sub.2 +NO.revreaction.N.sub.2 O.sub.3 PA0 (3) ROH+N.sub.2 O.sub.3 .fwdarw.RONO+HONO PA0 (4) ROH+HONO.fwdarw.RONO+H.sub.2 O PA0 (5) N.sub.2 O.sub.3 +H.sub.2 O.fwdarw.2HONO PA0 (6) 2NO.sub.2 .revreaction.N.sub.2 O.sub.4 PA0 (7) ROH+N.sub.2 O.sub.4 .fwdarw.RONO+HNO.sub.3 PA0 (8) N.sub.2 O.sub.4 +H.sub.2 O.fwdarw.HONO+HNO.sub.3
wherein R represents a methyl or ethyl group.
The desired reaction sequence for the formation of alkyl nitrite occurs via Reactions (1)-(4). The sum of these reactions yields as the overall process reaction: EQU 2ROH+2NO+1/2O.sub.2 .fwdarw.2RONO+H.sub.2 O I
Reaction (5) takes place because the water formed in Reaction (4) can react with dinitrogen trioxide (N.sub.2 O.sub.3). Reaction (5) can be tolerated provided enough alcohol is supplied to react with substantially all of the nitrous acid formed in Reaction (5) according to Reaction (4) yielding alkyl nitrite and additional water.
Reactions (6) through (8) are undesired since they lead to the formation of nitric acid, a compound which subsequently must be separated from product alkyl nitrite. Further, these reactions consume nitric oxide in forming undesired nitric acid. In order to reduce production of dinitrogen tetroxide (N.sub.2 O.sub.4), via Reaction (6), the gas phase concentration of NO.sub.2 should be minimized relative to that of NO. In this way, N.sub.2 O.sub.3 preferentially is formed instead of N.sub.2 O.sub.4. A relatively high NO to NO.sub.2 ratio can be maintained by initially supplying a molar excess of NO relative to O.sub.2, as indicated by the stoichiometry of Reaction (I), i.e., greater than 4 moles NO per mole O.sub.2. In other words, to enhance production of alkyl nitrites such as methyl nitrite or ethyl nitrite, it generally is preferable to provide NO in a molar excess, preferably in such an amount that substantially all O.sub.2 is consumed.
Vapor state formation of alkyl nitrite (nitrite process) by the general procedure described above preferably is coupled and correlated with vapor state formation of dialkyl oxalate from alkyl nitrite and carbon monoxide (oxalate process) in an integrated production cycle so as to provide an overall vapor state process (nitrite-oxalate process) that is cyclic in operation, e.g., see U.S. Pat. No. 4,629,806. Such a process is advantageous with regard to limiting the formation of by-products, ease of operation and production efficiency. Vapor state formation of dialkyl oxalate is conducted by contacting carbon monoxide and alkyl nitrite in a carbonylation reaction zone in the presence of a solid catalyst. The main reaction is illustrated by the following equation: ##STR1## wherein R represents a methyl or ethyl group.
Preparation of dialkyl oxalates is of particular interest to the chemical industry because of the varied used of these compounds. These diesters may serve as starting materials for the preparation of alkylene glycols such as ethylene glycol, a valuable commercial chemical which finds application in deicing fluids, antifreeze, hydraulic fluids and in the manufacture of alkyd resins, solvents, and polyester fibers. These diesters also are useful as intermediates in preparing dyes, pharmaceuticals, and the like.
As evident from the equation representing Reaction (II), for every mole of alkyl nitrite consumed, a mole of nitric oxide is generated. Nitric oxide thus formed may be recycled and used as a starting material for forming alkyl nitrites according to Reaction (I), thus completing the nitrite-oxalate reaction cycle. Dialkyl oxalate produced in the carbonylation reaction zone can be purified and recovered as product or further reacted, for example, by contacting it with hydrogen in a hydrogenation reaction zone to produce ethylene glycol.
Unless means are provided for removing various gaseous impurities from the nitrite-oxalate reaction cycle, however, the impurities gradually will increase in concentration in the recycle stream. These impurities include such gases as nitrogen, methane and carbon dioxide. One procedure commonly employed to curtail or counteract the buildup of inert impurities in a cycle is to withdraw or purge a small portion of a recirculating stream in the cycle and dispose of that portion, for example, to the atmosphere.
However, a purge stream taken from a nitrite-oxalate reaction cycle in accordance with the present invention will contain nitric oxide and alkyl nitrite in amounts which could have a harmful environmental effect. Nitric oxide is present as a result of using a molar excess thereof in the production of alkyl nitrite. In addition to potentially harmful effects on the environment, the loss of nitric oxide and alkyl nitrite by purging would represent a significant loss of valuable materials. Accordingly, nitric oxide and alkyl nitrite must be removed from any purge stream prior to discharging the impurities to the atmosphere.
A further consideration is that the approach taken should not result in formation of adverse amounts of materials deleterious to the integrated process. That is, in attempting to solve the problem presented by the need to purge impurities from a recirculation stream, which impurities are in admixture with otherwise valuable materials which themselves are potentially harmful to the atmosphere, it also is necessary to avoid forming and introducing materials into the integrated process which (1) adversely affect the formation of alkyl nitrite or dialkyl oxalate, and (2) reduce the economics of the process by requiring their subsequent separation from desired products. The present invention, then, is directed to a process for purging gaseous impurities from an integrated process while alleviating or avoiding these problems.